Serum HLA-A9, which is present in serum at 4 to 16-fold higher concentrations than other HLA antigens, is found in the high density lipoprotein fraction. By gel filtration on Bio Gel A-5m the molecular weight of the native antigen appears to be 62,000 daltons whereas that of the papain-digested antigen is 45,000 daltons. There is a system of alloantigens which are present on B lymphocytes but not T lymphocytes. These B alloantigens have been detected on the majority of acute and chronic leukemia cells including those of the myelocytic series. A rabbit anti B cell antisera raised to papain digests of malignant spleens from lymphoma patients appears to react against the common or non-variable region of the B alloantigenic molecule. The B antigen has a native molecular weight of approximately 65,000 daltons and it is composed of two polypeptide chains of 27,000 and 35,000 daltons. BIBLIOGRAPHIC REFERENCES: Billing, R.J., Rafizadeh, B., Drew, I., Hartman, G., Gale, R., and Terasaki, P.I. Human B lymphocyte antigens expressed by lymphocytic and myelocytic leukemia cells 1. Detection by rabbit antisera. J. Exp. Med. 144:167, 1976. Billing, R.J., Terasaki, P.I., Honig, R., and Peterson, P. The absence of B cell antigen B2 from leukemia cells and lymphoblastoid cell lines. Lancet i:365, 1976.